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Giraffe Evolution

Erik Kasarda Greg Kapustin Amanda Popek Courtney Ruhl

Outline

• Developmental Biology • Fossil evidence • Comparative biology • Physiology and physiological challenges • Necks for feeding hypothesis • Necks for Sex Hypothesis

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DEVELOPMENTAL BIOLOGY OF NECK ELONGATION

Developmental Biology Overview “Teenager Giraffes” • Same number of neck vertebrate as most other (7) • Elongation of the all cervical vertebras in a predictable way • Born with slightly longer necks • Cervical vertebrae begin to extend rapidly later in life (Van Sittert et al. 2010)

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Morphological Advantages • The cervical vertebrae close to the head are lighter than those lower in the neck • The occipital condyles allow for a wide range of motion • Massive sinus cavities

Developmental Sex Difference

• No sexual dimorphism in neck growth rate • Males have longer growth period • Larger heads and necks

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Fossil Evidence: Where are all the fossils? • Limited to a subset of ancestral forms: (Giraffa sp., boisseri/sinense, Paleotragus germaini, Paleotragus primaevus, Canthumeryx sirtensis, and Climacocerus gentryi)

• - a transitional fossil between extant giraffes and extinct shorter-neck giraffes

• Did blunt develop at the same time that neck elongation appeared?

Approximate Timeline • Significant neck elongation began around 14 million years ago • after the lineage to the split off • ~5 million years ago giraffes of modern proportions had evolved • The transition between the early, short-necked forms and the first long-necked giraffes • 12-14 million years ago • Global pattern of aridification

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Phylogeny

A. Hassanin et al., C. R. Biologies 330 (2007)

Comparative Anatomy- Okapi and Giraffes

• The family • • Okapi • Okapi’s show many features of fossils of early giraffe relatives • Okapi’s necks are still longer than most ruminants • Out-group analysis on the Okapi • Giraffe neck length has increased much more than Giraffe leg length

Photo by Howard Penn

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Giraffes did not evolve over night!

A long neck requires a special heart

Pumping Blood w/ Gravity?

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Loss of Oxygen?

Laryngeal Nerve

This nerve gradually evolved and lengthened, simpler than completely re-wiring

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Misunderstandings of evolution

Lamarck and giraffe evolution

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What is the pattern of giraffe evolution?

The process of giraffe evolution

• Niche definition is the role of giraffes on the African savanna ecology. This process is not an alternative to explaining giraffe evolution.

• Resource partitioning to avoid interference competition with other browser species that live in the same habitat.

• Giraffes have evolved and live in a bottom up habitat where availability of forage has direct impact on population size and species range.

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Giraffe browsing physiology and behavior.

Efficiency at browsing and neck Classic resource partitioning with position. other browser species.

Effect of giraffe browsing on its habitat

According to Moncrieff et al, browsing Over browsing causes high mortality in have affected distribution of trees that giraffe feed on. trees.

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Giraffe’s body size and predation

Red Queen at Work

Plant respond to browsing by evolving long Giraffe respond to plants thorns with a thorns and produce unpalatable tannins. prehensile tongue to access leaves.

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Other forms of defense by acacia trees from giraffe browsing

Acacia have evolved stipular spines to form a mutualistic relationship with ants as a defense Giraffe browsing behavior is modified against herbivores in general. by the aggressiveness of the ants.

Feeding Competition Working Alone?

• Provides compelling evidence

• Leaves unanswered: • Unequal neck lengths between sexes?

• Unequal elongation of all body parts?

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Feeding Competition Working Alone?

• Observational studies • No direct feeding advantage in present-day

• Very little partitioning with co- existing browsers

Feeding Competition Working Alone?

• Selection has occurred primarily on the neck

• Leg : Neck ratios

Okapi à 1 : 0.44 Male à 1 : 1.08 Female à 1 : 1.13

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Sexual Selection Hypothesis

• Predictions 1) Males should invest more in necks and skulls than females

2) Male giraffes should only use necks/heads for combat

3) Males with larger necks should be dominant and therefore gain a reproductive advantage

Necks - Size Matters

• Male Neck Length • Average 12-15 in. longer • 1.7x heavier

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Heads as Clubs

• Female Skulls • Smaller • Less armored • 3.5x lighter

Intrasexual Selection

• ‘Necking’ • Type of combat

• Unique to giraffes

• Establishes breeding status

• Occurs in both wild and captive individuals

• Females have never been seen engaging in ‘necking’ behavior

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Reproductive Advantage?

• Pratt and Anderson (1982) • Body size or neck size providing real advantage?

• Observational Study • Ranked wild males according to neck size and horn length (A-C)

à Larger-necked A bulls • consistently dominant • courted females significantly more often • females allowed urine testing by A bulls more often than B and refused C bulls in 2/3 of encounters

Necks for sex? Or necks for feeding?

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Discussion Questions

• A number of selection pressures have worked simultaneously on giraffe evolution. Discuss two possible factors that have contributed to the evolution of the long neck.

• Discuss two physiological adaptations related to the evolution of the giraffe neck.

• Explain why there are long-necked female giraffes.

References Bond W and Loffel D, 2001. Introduction of giraffe changes acacia distribution in a south african savanna. African journal of ecology. 39, 286-295.

Brown, D. M.; Brenneman R. A.; Koepfli, K-P.; Pollinger, J. P.; Milá, B.; Georgiadis, N. J.; Louis Jr., E. E.; Grether, G. F.; Jacobs, D. K.; Wayne R. K. (2007). "Extensive population genetic structure in the giraffe". BMC Biology 5 (1): 57. doi:10.1186/1741-7007-5-57.

Cameron E and Du Toit J, 2007. Winning by a Neck: Tall Giraffes Avoid Competing with Shorter Browsers. The American Naturalist, Vol. 169, No. 1, pp. 130-135.

Cameron, E.Z. & du Toit, J.T. (2005). Social influences on vigilance behaviour in giraffes, Giraffa camelopardalis. Behaviour. 69:1337–1344.

Mitchell, G., Roberts, D., van Sittert, S., & Skinner, J. (2013). Growth patterns and masses of the heads and necks of male and female giraffes. Journal of Zoology DOI: 10.1111/jzo.12013 . Philosophical Transactions of the Royal Society of London. Vol. 283, No. 996, pp. 165-229.

Goheen J, Palmer T, Keesing F, Riginos C, and Young T, 2010. Large herbivores facilitate savanna tree establishment via diverse and indirect pathways. Journal of Animal Ecology, 79, 372–382.

Hamilton W 1978. Fossil Giraffes from the of Africa and a Revision of the Phylogeny of the Giraffoidea. Philosophical Transactions of the Royal Society of London. Vol. 283, No. 996, pp. 165-229.

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Hopcraft J, Olff H and Sinclair A, 2009. Herbivores, resources and risks: alternating regulation along primary environmental gradients in savannas. Trends in Ecology and Evolution Vol.25 No.2 pp 119-128. http://en.wikipedia.org/wiki/Giraffe http://en.wikipedia.org/wiki/Okapi http://www.giraffeconservation.org/giraffe_facts.php?pgid=51

J. Exp. Zool. (Mol. Dev. Evol.) 314B:469–479, 2010. © 2010 Wiley-Liss, Inc. Martins D, 2010. Not all ants are equal: obligate acacia ants provide different levels of protection against mega-herbivores. Blackwell Publishing Ltd, Afr. J. Ecol., 48, 1115–1122.

Moncrieff G, Chamaille J, Higgins S, O’hara and Bond W, 2011. Tree allometries reflect a lifetime of herbivory in an African savanna. Ecology, 92(12), pp. 2310–2315.

O'Kane C, Duffy K, Page B and Macdonald D, 2011. Overlap and seasonal shifts in use of woody plant species amongst a guild of savanna browsers. Journal of Tropical Ecology / Volume 27 / Issue 03, pp. 249 258.

Pratt, D.M. & Anderson, V.H. (1982). Population, distribution, and behaviour of giraffe in the Arusha National Park, Tanzania. Journal of Natural History 16:481–489.

Radloff F and Toit J, 2004. Large predators and their prey in a southern African savanna: a predator’s size determines its prey size range. Journal of Animal Ecology. 73 pp. 410 –423

Simmons RE and Scheepers L. 1996. Winning by a neck: sexual selection inthe evolution of giraffe. The American Naturalist. 148(5): 771-786

Van Sittert, S. J.; Skinner, J. D.; Mitchell, G. (2010). From fetus to adult – An allometric analysis of the giraffe vertebral column. Journal of Experimental Zoology Part B Molecular and Developmental Evolution 314B(6):469–79.

Wilkinson D and Ruxton G, 2012. Understanding selection for long necks in different taxa. Biol. Rev., 87, pp. 616–630.

Young, T.P. & Isbell, L.A. (1991). Sex differences in giraffe feeding ecology: energetic and social constraints. Ethology 87:79–89.

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